Abstract
Observations of Kepler's supernova remnant (G4.5+6.8) with the H.E.S.S. telescope array in 2004 and 2005 with a total live time of 13 h are presented. Stereoscopic imaging of Cherenkov radiation from extensive air showers is used to reconstruct the energy and direction of the incident gamma rays. No evidence for a very high energy (VHE: >100 GeV) gamma-ray signal from the direction of the remnant is found. An upper limit (99% confidence level) on the energy flux in the range 230 GeV - 12.8 TeV of 8.6 x 10^{-13} erg cm^{-2} s^{-1} is obtained. In the context of an existing theoretical model for the remnant, the lack of a detectable gamma-ray flux implies a distance of at least 6.4 kpc. A corresponding upper limit for the density of the ambient matter of 0.7 cm^{-3} is derived. With this distance limit, and assuming a spectral index Gamma = 2, the total energy in accelerated protons is limited to E_p < 8.6 x 10^{49} erg. In the synchrotron/inverse Compton framework, extrapolating the power law measured by RXTE between 10 and 20 keV down in energy, the predicted gamma-ray flux from inverse Compton scattering is below the measured upper limit for magnetic field values greater than 52 muG.
Highlights
It is widely believed that the bulk of the Galactic cosmic rays (CR) with energies up to at least several 100 TeV originates from supernova explosions
An upper limit on the integrated energy flux above 230 GeV is derived. Combining this HESS result with the theoretical predictions of Berezhko et al (2006) suggests a lower limit on the distance, close to the upper limit given by Reynoso & Goss (1999), if Kepler’s SN is a priori assumed to be of type Ia
Observations of Kepler’s supernova remnants (SNRs) with HESS result in an upper limit for the flux of very high energy (VHE) gamma rays from the SNR
Summary
It is widely believed that the bulk of the Galactic cosmic rays (CR) with energies up to at least several 100 TeV originates from supernova explosions (see for example Drury et al 1994) This implies copious amounts of very high energy (VHE: >100 GeV) nuclei and electrons in the shells of supernova remnants (SNRs). Most recently Reynolds et al (2007) reported on deep Chandra observations and argued from the high abundance of iron and the very low abundance of oxygen that the progenitor of Kepler’s SNR has been a type Ia SN It appears that the observational evidence is converging on a type Ia event. Combining this HESS result with the theoretical predictions of Berezhko et al (2006) suggests a lower limit on the distance, close to the upper limit given by Reynoso & Goss (1999), if Kepler’s SN is a priori assumed to be of type Ia
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